Because the data rate from Mariner 4 to Earth was so slow, images were recorded on tape and replayed after the fly-by was complete. It took around six hours to transmit each of the 21 complete images captured during the Mars encounter.
On this note, it’s interesting that NASA Spirit and Opportunity were going to use a lossless wavelet image compression algorithm that ended up draining the little robots’ batteries. So NASA instead took an ISO standard (JPEG LS), programmed the core algorithm into FPGA and shipped it off. The technology ended up being retained even for Curiosity, even if the underlying raison d’etre no longer applied with Curiosity’s plutonium-based power system.